includes/Stg.h: add declarations for hs_popcnt and frinds

This fixes most of implicit function declarations emitted
C codegen in UNREG mode. Found by adding the following to
mk/build.mk:

    SRC_CC_OPTS += -Werror=implicit-function-declaration
    SRC_HC_OPTS += -optc-Werror=implicit-function-declaration

Issue #8748

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: Austin Seipp <austin@well-typed.com>